The present invention relates to a liquid crystal display device, and relates to a light source suitable for efficiently enhancing a brightness of a display screen and making it uniform over the entire display screen area, and a control method therefor.
A display devise using a liquid crystal display element (also called a liquid crystal display panel), an electroluminescent element (which is divided into an organic system and an inorganic system depending on a fluorescent material used, hereinafter referred to as an EL element), a field emission device (hereinafter referred to as an FE element) or the like displays an image without requiring a space (a vacuum envelope) for scanning an electron beam two-dimensionally on the back of the display screen as in a cathode ray tube (CRT). Accordingly, these display devices have characteristics that they are thin and light as compared with the CRT, and power consumption is low. These display devices are sometimes called a flat panel display because of its external appearance.
The display device using a liquid crystal display element, an EL element or a field emission device or the like has been widely spread due to the above-described advantage with respect to the CRT in place of a display device using the CRT in various uses. The fact that replacement from the CRTs to the flat panel displays has been progressed is also due to a technical innovation in enhancement of quality of images of a liquid crystal display element or an EL element. With the recent spreading of multi-media or the Internet, displaying of moving pictures has been strongly demanded. For example, in the display device using a liquid crystal display element, an improvement in a liquid crystal material or a driving method has been made for realizing a moving picture display. However, in the display device called a flat panel display as well as a display device using a liquid crystal display element, an increase of brightness is an important factor for displaying an image equal in quality to that of a conventional CRT.
For obtaining a moving picture display equal in quality to that of the CRT, it is essential to have impulse-type light generation as by scanning an electron beam projected from an electron gun on each pixel to excite phosphors of respective pixels to luminescence. On the other hand, for example, the liquid crystal display device utilizes the hold-type light generation using a backlight system by way of a fluorescent lamp, and therefore, complete moving picture display has been difficult.
The processes for solving the above-described problems in connection with the liquid crystal display devices reported are an improvement in a liquid crystal material for a liquid crystal display cell (a liquid crystal layer sealed between substrates) or a display mode, and a method for using a direct-light backlight (a light source construction for arranging a plurality of fluorescent lamps opposite to a display screen of a liquid crystal display element). FIG. 31 shows one example of a method of lighting of the direct-light backlight proposed for the moving picture display, using a layout of the direct-light backlight having eight (8) tubular lamps arranged opposite to a display screen (a frame indicated by the broken line), and timing of lighting-start time of the lamps provided thereon in terms of brightness waveforms. The brightness waveforms shown in FIG. 31 show that upward projections depict brightness rises.
As is apparent from FIG. 31, the lighting-start time of the respective fluorescent tubes is successively delayed from one fluorescent tube at the top to one fluorescent tube at the bottom. A series of lighting operation is synchronized with a scanning period of image display signals, and is repeated every image display period of one frame (a period for transferring video signals to all pixels of a display screen). (See xe2x80x9cLIQUID CRYSTALxe2x80x9d, Vol. 3, No.2 (1999), p. 99-p. 106.)
On the other hand, there is a technique for modulating brightness of a light source according to a scene of a moving picture signals transmitted to the liquid crystal display device. In this technique, a maximum brightness data, a minimum brightness data and an average brightness data of a video signal transmitted to the liquid crystal display device are read every image (in the case of a movie film, every xe2x80x9cframexe2x80x9d) constituting a moving picture frame to control a current (hereinafter called a lamp current) supplied to a light source according to the data. Suppose a current supplied normally to the light source is a reference current (for example, 4.5 mA), in the case of an image which is bright over the entire area, a lamp current is set to be higher than the reference current (for example, 8 mA) in a certain period, and is returned to the reference current later. Conversely, in the case of an image which is dark over the entire area, a lamp current is set to be lower than the reference current (for example, 1.5 mA). (See xe2x80x9cNIKKEI ELECTRONICSxe2x80x9d, Nov. 15, 1999 issue, No. 757, 1999, p139-p146)
In the case of the former (the wholly bright image), temperature rise of a light source is larger by a portion corresponding to an increase in current supplied to the light source from the reference current. In the case of a fluorescent lamp, vapor pressure of mercury (Hg) within a fluorescent lamp rises due to rising of temperature thereof, and mercury atoms (the amount of mercury vapor) increase within the fluorescent lamp. On the other hand, surplus mercury atoms are present within the fluorescent lamp, there is increased probability that ultraviolet rays produced within the fluorescent lamp due to collision between hydrogen atoms and electrons are absorbed by the mercury atoms, and brightness of the fluorescent lamp decreases. For avoiding this influence, a lamp current is set to be higher than the reference current in the period described above, after which the lamp current is returned to the reference current before the mercury vapor pressure within the fluorescent lamp changes. By changing the lamp current as described above, the brightness of the fluorescent lamp is made higher than that when the reference current is supplied thereto.
In the case of the latter (the wholly dark image), when the brightness of the light source is high, it is necessary to suppress a leakage of a small amount of light from a pixel which displays black or a color close thereto. In the wholly dark screen, even for the pixel whose light transmission is set to be highest within the screen, the absolute amount of light to be transmitted is small. Because of this, the lamp current is set to be lower than the reference current, and the brightness of the light source is suppressed to restrict leakage of light from a pixel which displays black or color close thereto, and power consumption in the light source is reduced.
From a combination of the two techniques, the dynamic range of brightness (the ratio of the maximum brightness to the minimum brightness) in the moving picture image as a whole becomes 2.8 times that of the conventional one, and the contrast ratio is from 400:1 to 500:1, which is not less than 2 times that of the conventional liquid crystal display device.
In a case where in the liquid crystal display device, the technique of lighting light sources in turn in the direct-light backlight as described above is carried out, if the number of lamps (fluorescent lamps) mounted on the direct-light backlight is increased, for example, a light-generating duration of each lamp during a lighting operation of one period (corresponding to one frame) should be shortened. Because of this, the brightness efficiency of the whole direct-light backlight lowers.
On the other hand, when power applied to each lamp is increased in order to raise a brightness of a display image, a liquid crystal cell is locally heated by heat generation of the lamp, and display uniformity also lowered. An image display in the liquid crystal display device is carried out by twisting a liquid crystal molecule sealed in a liquid crystal cell of a liquid crystal display element mounted in a direction corresponding to the image information (field applied to a liquid crystal cell), and changing the light transmission to the desired value. For twisting the liquid crystal molecule within the liquid crystal cell decidedly in a direction in response to the image information, a chiral agent together with the liquid crystal molecule is sometimes incorporated into the liquid crystal cell. A layer of substances which are present within the liquid crystal cell including these additives is sometimes called xe2x80x9ca liquid crystal layerxe2x80x9d. However, when a temperature of the liquid crystal cell locally rises, the light transmission of the liquid crystal cell varies in that portion according to the change in the refractive index of the liquid crystal molecule present in that portion, and therefore, non-uniformity occurs in the display image. Further, the viscosity of the liquid crystal layer lowers in that portion, and the directions of a portion of the liquid crystal molecules becomes random (the liquid crystal layer becomes isotropic). Accordingly, the light transmission of a part of the liquid crystal cell fails to correspond to the electric field applied to the liquid crystal molecules, and the display non-uniformity described above occurs. This problem often occurs, as compared with a liquid crystal display device of a twisted nematic type (TN type), in a vertical alignment type (VA type) in which a temperature at which the liquid crystal layer becomes isotropic is low (which is called a transition temperature of liquid crystal material or a transition temperature), or in a liquid crystal display device of the horizontal electric field type (the in-plane-switching type, or the ISP type). Therefore, it is difficult to increase the display brightness of the ISP type liquid crystal display device.
Further, in a case where the technique of adjusting the brightness of a light source for every image formed by moving picture signals is applied to the liquid crystal display device, setting of timing for reduce a large lamp current of the light source for the wholly bright image to the reference current is difficult in practical use. As described above, for increasing the brightness of the light source from that provided by the reference current, initially the lamp current has to be made higher than the reference current, and then the lamp current has to be returned to the reference current before the mercury vapor pressure within the fluorescent lamp changes. However, timing for changing the lamp currents has to be set experientially, for example, on the basis of correlation between measured data of changes in temperature of the light source (the fluorescent lamp) and the brightness of the light source. Further, since in this technique, the light source brightness at the respective image display time is changed according to the brightness of the respective image, the contrast ratio for every image remains the value that may be achieved by the conventional liquid crystal display device. In other words, even if this technique is applied to the liquid crystal display device, in a case where an image whose brightness rarely varies for a given period (a period in which a plurality of image data are transmitted to the liquid crystal display device) such as a static image is displayed, the contrast ratio cannot be enhanced.
The relationship between the current supplied to the light source of the liquid crystal display device and the temperature of the light source or the brightness is discussed, for example, in Japanese Publications such as Japanese Patent Laid-Open Nos. Hei 11-38381 (laid-open on Feb. 12, 1999), 9-260074 (laid-open on Oct. 3, 1997), 11-283759 (laid-open on Oct. 15, 1999), 7-175035(laid-open on Jul. 14, 1995), and 8-8083 (laid-open on Jan. 12, 1996). However, even if these publications are referred to, it is difficult to find the conditions for adequately setting switching timing of the lamp current.
An object of the present invention is to provide a liquid crystal display device by which brightness of an image displayed on a liquid crystal display panel (a liquid crystal display element) mounted thereon is efficiently improved, and various problems associated with heat generation of a light source for illuminating the liquid crystal display panel with light are solved.
A further object of the present invention is to provide a liquid crystal display device by which an image or video is displayed with a contrast ratio as high as that of a CRT.
For achieving the aforementioned objects, the present invention provides liquid crystal display devices configured as mentioned below.
In accordance with an embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; and a circuit for supplying alternately a first alternating current of a first amplitude during a first period t1 and a second alternating current of a second amplitude during a second period t2 to the at least one light source, the first amplitude being greater than the second amplitude, wherein the circuit controls the first alternating current and the second alternating current such that the following relationship is satisfied: first electric power E1 is lower than second electric power E2, where the first power E1 is defined as {(t1xc3x97ip-p(1)xc3x97Vp-p(1))/2}+{(t2xc3x97ip-p(2)xc3x97Vp-p(2))/2}, ip-p(1)=a peak-to-peak value of the first alternating current flowing through a respective one of the at least one light source during the first period t1, Vp-p(1)=a peak-to-peak value of a voltage across the respective one of the at least one light source during the first period t1, ip-p(2)=a peak-to-peak value of the second alternating current flowing through the respective one of the at least one light source during the second period t2, Vp-p(2)=a peak-to-peak value of a voltage across the respective one of the at least one light source during the second period t2, the second electric power E2 is defined as (t1+t2)xc3x97(Ieffxc3x97Veff), Ieff is an effective value of a current flowing through the respective one of the at least one light source during the first period t1 plus and the second period t2, and Veff is an effective value of a voltage across the respective one of the at least one light source during the first period t1 plus and the second period t2.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; and a circuit for alternately supplying a lamp current to the at least one light source during a first period t1 and ceasing to supply the lamp current to the at least one light source during a second period t2, wherein the following relationship is satisfied: first electric power E1 is lower than second power E2, where the first power E1 is defined as (t1xc3x97ip-pxc3x97Vp-p)/2, ip-p=a peak-to-peak value of the lamp current flowing through a respective one of the at least one light source during the first period t1, Vp-p=a peak-to-peak value of a voltage across the respective one of the at least one light source during the first period t1, the second power E2 is defined as (t1+t2)xc3x97(Ieffxc3x97Veff), Ieff is an effective value of the lamp current flowing through the respective one of the at least one light source during the first period t1 plus and the second period t2, and Veff is an effective value of a voltage across the respective one of the at least one light source during the first period t1 plus and the second period t2.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; and a circuit for supplying alternately a first voltage having a first effective value V1 during a first period t1 and a second voltage having a second effective value V2 during a second period t2 to the at least one light source, the first voltage producing a first current having a first effective value i1 flowing through the respective one of the at least one light source during the first period t1, the second voltage producing a second current having a second effective value i2 flowing through the respective one of the at least one light source during the second period t2, the second effective value i2 being smaller than the first effective value i1, wherein a first ratio of a first brightness to a first electric power is greater than a second ratio of a second brightness to a second electric power, where the first brightness is a brightness produced by the respective one of the at least one light source during the first period t1 plus the second period t2, the first electric power is defined as {(t1xc3x97V1xc3x97i1)+(t2xc3x97V2xc3x97i2)}/(t1+t2), the second electric power is defined as (Veffxc3x97ieff), Veff is an effective value produced by a combination of the first voltage supplied during the first period t1 and the second voltage supplied during the second period t2, ieff is an effective value produced by a combination of the first current flowing during the first period t1 and the second current flowing during the second period t2, and the second brightness is a brightness produced by the respective one of the at least one light source supplied with the second electric power.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having a cold-cathode tube having an end-to-end length L (cm) and projecting light generated by the cold-cathode tube on the liquid crystal display panel; and a circuit for supplying alternately a first electric power W1 (W) during a first period t1 and a second electric power W2 (W) during a second period t2 to the cold-cathode tube, the second electric power W2 being lower than the first power W1, wherein a ratio of the first electric power W1 to the end-to-end length L is greater than 0.2 W/cm, and a ratio of the second electric power W2 to the end-to-end length L is equal to or less than 0.1 W/cm.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; and a circuit for supplying alternately a first current having a first effective value i1 during a first period t1 and a second current having a second effective value i2 during a second period t2 to a respective one of the at least one light source, the first effective value i1 being greater than a rated value of a lamp current flowing through the respective one of the at least one light source, the second effective value i2 being smaller than the rated value of the lamp current, wherein the first effective value i1, the second effective value i2, the first period t1 and the second period t2 are selected such that an integral of brightness produced by the respective one of the at least one light source over the first period t1 plus the second period t2 is greater than an integral of brightness produced by the respective one of the at least one light source supplied with the lamp current of the rated value over the first period t1 plus the second period t2.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; a control circuit configured so as to switch between a first operation and a second operation, the first operation supplying a first current having a first effective value i1 during a first period t1 to the at least one light source, and the second operation supplying a second current having a second effective value i2 during a second period t2 to the at least one light source, the second effective value i2 being smaller than the first effective value i1; and a temperature detector circuit for detecting a temperature of the at least one light source, wherein the temperature detector circuit transmits a signal to the control circuit when a temperature of an outside wall of the at least one light source exceeds 65xc2x0 C., and the control circuit switches from the first operation to the second operation in response to the signal.
In accordance with another embodiment of the present invention, there is provided a liquid crystal display device comprising: a liquid crystal display panel having a plurality of pixels; a lighting device having at least one light source and projecting light generated by the at least one light source on the liquid crystal display panel; a control circuit configured so as to switch between a first operation and a second operation, the first operation supplying a first current having a first effective value i1 during a first period t1 to the at least one light source, and the second operation supplying a second current having a second effective value i2 during a second period t2 to the at least one light source, the second effective value i2 being smaller than the first effective value i1; and a brightness detector circuit for detecting brightness of the at least one light source, wherein the brightness detector circuit transmits a signal to the control circuit when the brightness begins to reduce in the first period t1, and the control circuit switches from the first operation to the second operation in response to the signal.